1. Field of the Invention
The present invention relates to an AC-DC converter which is designed to realize an improved power factor and thereby minimize generation of voltage distortion and higher harmonics in power supply line.
2. Description of the Prior Art
Conventional AC-DC converter usually uses, as its input circuit, a condenser input type rectifier circuit as shown in FIG. 1, which comprises a rectifier 1 connected to commercial power supply line, a smoothing capacitor C.sub.1 connected to the output terminal of the rectifier 1, a converter transformer 2, and a switching transistor Q.sub.1 connected in series with the output terminal of the rectifier 1 and a primary winding L.sub.1 of the converter transformer 2.
Connected to a secondary winding L.sub.2 of the transformer 2 is a rectifying-smoothing circuit which comprises a rectifier diode D.sub.1, flywheel diode D.sub.2, choke coil L.sub.3, and smoothing capacitor C.sub.2.
With such an AC-DC converter, a signal derived from a pulse width modulator circuit (not shown ) is supplied to the base of the switching transistor Q.sub.1 to control the "on" time of the switching transistor so that a stabilized DC output is obtained across output terminals 4A and 4B. 3A and 3B indicate input terminals of the rectifier 1 which are connected to the commercial power supply line, respectively.
FIG. 2 illustrates voltage and current waveforms of the AC-DC converter shown in FIG. 1, wherein v.sub.1 indicates the output waveform of the rectifier 1, and i.sub.1 shows the current waveform of the rectifier 1 which consists of only the positive-going components of the input current waveform of the AC-DC converter, which input current waveform consists of alternate positive -going and negative-going components. In FIG. 2, the waveform shown mainly by dotted lines is the output voltage waveform of the rectifier 1 which occurs in the absence of the smoothing capacitor C.sub.1.
With the AC-DC converter including the condenser input type rectifying circuit, the output current i.sub.1 of the rectifier 1 takes such a waveform that a major portion thereof passes through the capacitor C.sub.1 for a short period of time and thus the flow angle thereof is very small.
As will be seen from the above explanation, the above-mentioned conventional AC-DC converter Is disadvantageous in that it provides a low power factor and is liable to generate voltage distortion and higher harmonics which occur in the power supply line. Such voltage distortion and higher harmonics will have adverse effect on other electronic equipments or the like through the commercial power supply line, and are becoming social problems today since electronic equipments such as robot or the like which must be prevented from any erroneous operation by any means are increasing in number.
FIG. 3 is a circuit diagram of another conventional AC-DC converter, parts of which correspond to those of FIG. 1 are indicated by like reference numerals or symbols. This AC-DC converter includes an active filter 5 connected to the output terminal of the rectifier 1.
The active filter 5 comprises a diode D.sub.3 and choke coil L.sub.4 which are connected in series with each other between the positive side output terminal of the rectifier 1 and the primary winding L.sub.1 of the converter transformer 2; a transistor Q.sub.2 connected between the connection of the choke L.sub.4 and diode D.sub.3 and the negative side output terminal of the rectifier 1; an output capacitor C.sub.3 connected between the connection of the diode D.sub.3 and primary winding L.sub.1 and the (-) side output terminal of the rectifier 1; and a control circuit 6 for the transistor Q.sub.2.
The active filter 5 is arranged such that a current proportional to the output voltage of the rectifier 1 is caused to flow through the choke coil L.sub.4 by detecting the voltage at the output capacitor C.sub.3 and the output voltage of the rectifier 1, and by controlling the transistor Q.sub.2 by means of the control circuit 6 which operates with the detected voltage.
FIG. 4 illustrates the voltage and current waveforms of the AC-DC converter shown in FIG. 3, wherein the output voltage and output current of the rectifier 1 are represented by v.sub.2 and i.sub.2 respectively. The output current i.sub.2 is represented as average value.
With the AC-DC converter of FIG. 3, the flow angle of the output current i.sub.2 of tile rectifier is extended so that the power factor is improved and generation of the voltage distortion and higher harmonics in the power supply line is reduced.
However, the AC-DC converter shown in FIG. 3 is disadvantageous in that the entire circuit arrangement thereof becomes complicated due to the fact that the active filter 5 requires the bulky choke coil L.sub.4 and a multiplier circuit (contained in the control circuit 6). The choke coil L.sub.4 becomes bulky as the power to be handled increases.